Synthetic rutile crystal and method for making same



May 14, 1957 c. H. MOORE, JR., ETAL 2,792,287

SYNTHETIC RUTILE CRYSTAL AND METHOD FOR MAKING SAME Filed April 4, 1956(S 5 R Y E m R M m m m Jm m Aw v a m R w m m m L 0 H w m f Q m m a R L jm 5 0 H 2 G a 1 Y HJ B w A 1 mm. my E Hwmv Hm o C 5 E m fl/ m 4 1 a 6 78 2 2 M a Y w m K F SYNTHETIC RUTILE CRYSTAL AND METHOD FUR MAKING SAWCharles H. Moore, (in, Indianapolis, Ind, and Roy Dfilll strum,Westfield, N. 5., assignors to National Lead Company, New York, N. Y., acorporation of New Jersey Application April 4, 1956, Serial No. 576,161

12 Claims. (Cl. 23-402) The present invention relates to rutile crystalsof large size and of uniform predetermined color and quality and to theproduction of such crystals.

This application is a continuation-in-part of applications Serial Nos.54,562 and 54,563 filed October 14, 1948, both now abandoned, andapplication Serial No. 286,853 filed May 9, 1952, which also has beenabandoned.

The term boule as used herein is a term of art used to denote acharacteristic crystal form or shape and is applied particularly to thesynthetic crystals of the form produced by processes similar to theprocess of Verneuil.

Rutile is one of the three crystal modifications of titanium dioxide. Innature, rutile generally occurs as tetragonal crystals which are oftengeniculated and twins are common. Rutile also sometimes appears as thin,hairlike crystals in quartz. Nowhere in nature does rutile occur as 'aboule nor does rutile occur in nature as a straw-white single crystal.

When substantially pure, a single crystal of rutile has gem-likeproperties with a very light straw color and reflectance, refraction andbrilliance greater than that of diamonds. Its optical properties suggestits usefulness in the form of prisms, lenses and the like for opticalinstruments. When slightly deficient in oxygen, the color of pure rutilevaries through shades of blue to blue-black. The color and otherproperties of rutile single crystals can also be varied by the inclusionof additives such as metal salts.

The art of growing synthetic crystals is quite old and well known, butprior to the present invention it was impossible to produce syntheticcrystals of rutile by known methods. In the course of the researchesleading to the present invention, early attempts to prepare rutilesingle crystal boules by the Verneuil process met with no successlargely because certain factors peculiar to rutile were not understoodand appreciated.

Titanium dioxide has a strong tendency to give up oxygen at elevatedtemperatures and form lower oxides of titanium. We have found that atthe high temperatures necessary to grow single crystal rutile boules(about 1825 C. to 1900 0), this tendency is so strong that unless anoxidizing flame is used, there is strong likelihood that some of theTiOz will be changed to a lower oxide such as TizOa with the result thatthe boule will be multicrystalline rather than a single crystal.

Even when the boule is formed under properly oxidizing conditions, it isusually blue-black with a metallic luster when broken, indicating anoxygen deficiency. We have found that this blue-black color can beeliminated by oxidizing the boule by heating it in an oxidizingatmosphere. When a single crystal of substantially pure rutile issatisfied with respect to oxygen, that is to say contains stoichiometricproportions of oxygen and titanium, the color is nearly water-white witha very slight straw colored tone. As oxygen is restored to asubstantially pure rutile single crystal having the deep blue-blackmetallic color of the boule as prepared, the color grad- Patented May14,

ually lightens, passing through a range of color graduations which maybe described as deep blue, medium blue, light blue, etc. until thenearly water-white color of a rutile crystal completely satisfied withrespect to oxygen is achieved. The oxygen deficiency of bluemonocrystalline rutile is so slight that the crystal has both theapparent chemical composition and crystal structure of rutile and atpresent, no methods are known by which the actual oxygen deficiency canbe estimated quantitatively in terms of some standard of measure, e. g.weight or volume. It is evident, however, that the blue color effectresults from oxygen deficiency and very small variations of thisdeficiency produce definite and appreciable color differences. Thebluecrystals of substantially pure rutile produced according to thepresent invention may be cut and polished to form beautiful brilliantblue gems.

The properties of rutile single crystals can also be varied in otherways in addition to the control of oxygen content. Metal salts can beadded to the powdered TiOz starting material to give the resultingcrystal desired color or other properties. With such additives present,the boule is formed in the same way in an oxidizing flame, and then theboule or crystal is oxidized in the same way to reduce or eliminate theblue color and bring out the characteristic color due to the additive.For example, when the TiOz starting powder contains 0.04% FezOa, amonocrystalline boule can be formed. The boule when formed usually hasthe blue-black metallic luster resulting from oxygen deficiency. Heatingthe boule, or crystals cut from the boule, in an atmosphere of oxygen,causes the blue color to disappear and the boule, when fully oxidized,has a yellow color resulting from that amount of FezOs. When largeramounts of FezOs are added, the color of the fully oxidized boulebecomes darker and a boule having 0.2% FezOs is clear reddish color.Additions of other metal compounds such as cobalt or nickel havecoloring effects similar to iron. With one or more of these additives,boules can be made ranging in color from pale yellow to deep red, largeramounts of the additive giving the deeper color, but in any event theamount of coloring agent is small. Cobalt and nickel are especiallyuseful additives giving clear and pleasing colors from pale yellow,through amber, reddish amber and red to a deep reddish black. U. S.Patent No. 2,715,071, issued August 9, 1955 to Leon Merker describes andclaims specific amounts of various additives to secure particularcolors.

When very finely powdered alumina is added to the TiOz startingmaterial, fully oxidized crystals have a clear Water white color and thehardness of the crystals is increased. Satisfactory boules have beenmade with small amounts of alumina from 0.005% to 0.1%. U. S. Patent No.2,715,070 issued August 9, 1955 to Charles H. Moore, J r. describes andclaims rutile crystals containing alumina.

Regardless of whether the boule is to be formed of substantially purerutile or contains a coloring or other modifying agent such as alumina,the boule is formed and its oxygen content controlled in the same way.According to the present invention, any desired degree of oxygendeficiency as indicated by the depth or tint of the blue coloration, maybe obtained by stopping the oxidation, or by deoxidizing the crystal, tothe desired point.

One of the principal objects of the present invention is to provide arutile single crystal boule.

Another object is to produce rutile single crystals of such large sizethat gems and optically useful articles can be prepared therefrom.

Still another object is to provide a method for producing rutile singlecrystals which may be formed into beautiful, brilliant gems.

Another object is to provide rutile crystals having predeterminedproperties.

A further object is to provide a method for controlling the oxygencontent of rutile single crystals.

A further object is to provide a method for restoring the oxygen contentof oxygen deficient rutile single crystals.

A further object is to provide a novel process for converting darkopaque as-grown rutile crystals to a transparent and substantiallycolorless condition for use as gemstones.

A further object of the invention is to provide rutile single crystalshaving predetermined oxygen content, color and properties.

These and other objects and advantages reside in novel features andsteps and processes as hereinafter more fully set forth.

Figure 1 illustrates the characteristic shape of a boule.

Figure 2 is a schematic representation of a preferred form of apparatusfor carrying out the present invention.

Figure 3 is a cross-section of the burner of the apparatus of Figure 2taken on the line 3-3 of Figure 2.

Figure 4 is a schematic representation of the flame formation of theboule according to the present invention.

Figure 5 is an enlarged section of the top, or meniscus end of a bouleshowing the various temperature zones during formation.

A typical boule is illustrated in Figure l of the drawing. The bouleforms at a point 9, then tapers outwardly in somewhat conical form 10,then has a substantially cylindrical body portion 11 and finally has arounded end 12. The diameter and length of the body portion 11 can bevaried to produce boules of different sizes.

The first factor which must be considered for the successful practice ofthe present invention is the purity of the titanium dioxide startingmaterial. It is essential that the starting material be free, or atleast substantially free from components which prevent or inhibit thecrystallization of the TiOz in a single crystal of rutile. Suchinterference may result from crystalline structure or chemical action ofthe component.

For example, the TiOz starting material should be free or substantiallyfree from elements possessing ionic radii incompatible with the rutilecrystal lattice. The ionic radii of tetravalent titanium is reported tobe 0.68 Angstrom unit. It has been found that cationic elements havingionic radii less than about 0.60 Angstrom unit and greater than about0.75 Angstrom unit, and which are nonvolatile at the temperature of theboule formation, should not be present in the TiOz starting material inamounts substantially greater than a mere trace detectable only as suchby spectrographic analysis. Elements which have ionic radii betweenabout 0.60 and 0.75 Angstrom unit may be present in somewhat more thanspectrographically detectable traces. These enter the crystal latticestructure ofthe rutile single crystal to form solid solutions, whereaselements which have ionic radii outside the range specified, inhibit theformation of the rutile single crystal.

Furthermore, the starting material should be free, or at leastsubstantially free from elements which react with titanium or titaniumdioxide toform chemical compounds. Elements which react with titanuim ortitanium dioxide to form chemical compounds also inhibit the formationof rutile single crystals, regardless of whether such elements haveionic radii within the proper range. Thus, magnesium which has an ionicradius within the specified range cannot be used because it reacts withTiOz to form magnesium titanate. The compounds formed from such reactiveelements crystallize in their own distinctive patterns and inhibit theformation of a rutile single crystal.

Impurities which commonly occur in titanuim dioxide but which areincompatible with the rutile crystal lattice are silicon, magnesium andlead. With the techniques and equipment available when the presentinvention was made, these impurities, in general, could not be toleratedin amounts substantially greater than 0.15% silicon, calculated as SiOz;0.005% magnesium, calculated as MgO; and, 0.002% lead, calculated asPbO. In this connection, however, it should be pointed out that smallamounts of other elements which may be present in the TiOz startingmaterial as tolerated impurities, or added to impart a desired color tothe final rutile single crystal, may act as solvents for elements whichare undesirable, thereby raising the minimum amount of such undesirableimpurities which may be present without preventing formation of therutile single crystal boule. Thus, elements of the iron family such ascobalt and nickel, which impart to the final rutile crystal definitecolorations, will tend to raise the tolerance for silicon. It will beappreciated that a maximum upper limit for the content of incompatibleimpurities cannot be precisely given, but for the production of a rutilesingle crystal boule having maximum purity and a minimum of internalstresses, the TiOz starting material should be free, or at leastsubstantially free from incompatible impurities, as explained above. Asatisfactory TiOz starting material for the practice of the presentinvention should not contain more than about three-tenths per cent totalincompatible impurities. With improved techniques and equipment, largerpercentages of impurities may possibly be tolerated.

Some elements which may be added to the TiOz starting material for aparticular purpose may be undesired impurities under other conditions.For example, when a water-white crystal is desired, alumina is added toeliminate the straw-white tone of pure rutile. Even a trace of coloringoxide would be an undesirable impurity in such a case. Likewise, when acolored crystal is desired, alumina, which tends to lighten the color,may be undesirable.

It is, therefore, preferable that the TiOz starting material be as pureas is practically possible and all impurities should be eliminated orheld to a minimum regardless of the ionic radius or chemical or physicaleffect of. the impurity. When the TiOz starting material is free orsubstantially free of impurities, the operations employed in producingthe crystal can be standardized and the resulting crystals will besubstantially uniform as to color and structure.

Patent No. 2,521,392. issued September 5, 1950, discloses a suitableform of TiOz starting material together with method for its preparation.

For the most etficient results, the TiOz starting material should bevery fine, fairly uniform and possess an open structure with unitscapable of being rapidly melted. A TiO2 starting material having anultimate unit particle size of approximately 0.1 micron has provedespecially satisfactory. In general, material having an average particlesize above about 0.5 micron should be avoided because such particles donot satisfactorily fuse under the conditions of the invention. Additivessuch as coloring oxides should be used in powdered form and have aboutthe same particle size as the TiOz.

Another factor which the successful achievement of the present inventiondepends is the nature of the flame in which the particles of startingmaterial are fused. Verneuil processes employ a. flame resulting fromthe combustion of hydrogen and oxygen and in the prior art the ratio ofhydrogen to oxygen was such that the flame was reducing in varyingdegrees of intensity.

At elevated temperatures titanium dioxide 'ves up oxygen and unlessox"gen loss is prevented, all or part of the TiOz is converted to loweror suboxidcs of titanium, e. g. TizOs. This loss of oxygen proceed withconsiderable rapidity-under the conditions produced by an oxyhydrogenflame wherein the TiOz is fused and the loss'is accelerated when, as isthe case usin a typical Verneuilprocess, the flame is a reducing one.Under such conditions, the formation of lower or sub-oxides of '5titanium is such that mixtures of small crystals of various oxides oftitanium are produced rather than a single crystal boule.

The small particle size of the starting material and the prevention ofreduction during crystallization in accordance with our inventionresults in single crystal, i. e. massive non-granular boules. In thegrowing of single crystals, it seems to be important that the crystal begrown from a single nucleus, as a plurality of nucleii apparently resultin a multicrystalline rather than a monocrystalline structure. When theparticle size of the starting material is very small, the material isprobably fully melted under the conditions of the present process andrecrystallizes in the plane of the nucleus. Should the particles be toolarge to be fully melted, the unmelted portion might form a new crystalnucleus so that a multicrystalline structure would result. The same isprobably true if conditions are such that some TiaOs or TiO is formed.These compounds would break up the orderly formation of monocrystallineTiO2 so that new crystal nucleii would form and a multicrystalline masswould result.

It has been found according to the present invention that for theproduction of a rutile single crystal boule an excess of oxygen, overthat required completely to react with the hydrogen, should be fed tothe flame at all times. Preferably, the ratio of oxygen to hydrogen byvolume should be about 1:1 and should not be less than about 8.5 partsof oxygen to 9.5 parts hydrogen by volume. Preferably, also, a part andgenerally a major part of the oxygen should be fed to the flame as anouter cover or envelope.

The preferred form of apparatus for carrying out the present inventionis a modified Verneuil burner as illustrated in Figure 2 and the methodof carrying out the invention in its most effective embodiment will nowbe described.

The starting material 13, which may be substantially pure TiOz with orwithout a suitable additive, is placed in the hopper 14, which isprovided with a screen, preferably about 100 mesh, of wire mesh or silkcloth 15. The hopper 14 is mounted within a housing 14a and there is afree space between the hopper and the wall of the housing. The housingis provided with an inlet 16 for the introduction of oxygen, a cover 17and a knocker 18 connected with suitable electrical or mechanical means,for instance, for causing the knocker 19 periodically to rise and tofall, striking the cover 18. From the conical bot tom 19 of the housing14a below the hopper 14, a long tube 20 extends downward and forms atthe bottom, the inner concentric ring of the burner. This burner 21 isformed of the central tube 20, a jacket 22 forming a middle, orintermediate, concentric ring of the burner, provided with an inlet 23for the introduction of gas, and

a jacket 24 forming an outer concentric ring of the burner 1 providedwith a gas inlet 25. Generally we prefer to introduce oxygen through thecenter tube 20 and outer ring 24 and to admit hydrogen through theintermediate ring 22, but the burner may be operated by admitting oxygenthrough the center tube and intermediate ring and the hydrogen throughthe outer ring. If desired, one or more cooling jackets may be provided,for instance between the gas introduction jacket and around the outsideof the burner.

The burner opens into a chamber 26, which is preferably formed ofsuitable fire resistant material, such as firebrick, 27, convenientlyformed of two substantially semi-cylindrical halves. In the chamber 26there is positioned a rod 23 made of fire resistant material, e. g.firebrick or zirconia, upon which the flame impinges When the apparatusis operating and upon which the boule forms. The rod 23 is supported onplatform 29 provided with means, for example a threaded, screw withhandle 30, for lowering the platform 29 and rod 28 as the boule growsvertically in size.

In operating the apparatus, oxygen is admitted to the burner throughinlets 16 and 25, inlet 16 taking a minor portion of the oxygen, e. g.about one-quarter of total oxygen and inlet 25 the major portion, e. g.about threequarters. Hydrogen is admitted through inlet 23. It will beunderstood that the quantity of the respective gases and the rate offlow will be controlled by manometers and reducing valves, as necessary,and in known manner. The flame is then lit and the knocker 18 set inmotion. With each blow of the knocker, a small amount of startingmaterial is sifted through the screen 15 and entrained in the stream ofoxygen coming into the housing 14a through inlet 16 and carried throughthe tube 20 into the flame. The quantity of oxygen and hydrogen and therange of flow will be regulated during the formation of the boule toproduce a flame having a temperature somewhat higher than the meltingpoint of titanium dioxide which is about 1820 C. The flame should bekept at a temperature between about 1825 C. and 1900" C., preferably atabout 1850" C. and should not exceed 1900 C. because at this temperaturethe boule melts and flows over.

At the onset of preparing a boule, the flame is adjusted to atemperature slightly below the melting point of TiOz The particles ofTiOz are heated as they enter the flame and fall upon the top of the rod28 and form a fused sintered mass in the shape of a cone. The flametemperature is then increased to above the melting point of TiOz and thetop of the sintered cone becomes fused and forms a single crystal seedor bud which grows into a small sphere as additional heated TiOzparticles strike it and are fused. The top of this sphere is molten toslight depth, the molten portions being meniscus shaped as shown inFigure 5. The heated TiOz particles melt and form this molten meniscus.The single crystal or sphere grows upward vertically and horizontallythrough the solidification of the bottom of the molten mass. Thespherical shape is lost and the single crystal begins to assume thetypical shape of a boule. When the single crystal grows to its maximumdiameter, which is a function of the size of the flame, which is, inturn, a function of the dimensions of the burner, the growth of theboule is substantially all vertically upward. As the boule grows upward,the rod 28 and the platform 29 are gradually lowered by adjusting means30 so that the molten meniscus of the boule occupies the same positionin the flame. While growing, the boule has a molten meniscus at the topand temperature zones substantially as shown in Figure 5.

The operation may be carried on to obtain boules of any desired lengthwithin the maximum permitted by the dimensions of the apparatus. It hasbeen found that with increasing diameter of the boule, the internalstrains and stresses tend to increase. With a burner of the characterdescribed with a nozzle of three-quarters inch wide, it has been foundentirely practicable to produce boules having diameters between aboutone-half to threequarters inch. The length of the boule appears to haveno effect on internal strains, but the longer the boule becomes, thegreater is the strain on its tip or seed end 9, and if the boule becomestoo long, it will snap off from the rod 28. With boules having diametersbetween one-half and three-quarter inch, it has been found possible tomake boules of from one and one-half to two inches in length. It will beunderstood that the invention is not limited to boules of any particulardiameter and length, but may be operated, if necessary, with suitablymodified apparatus to produce boules of any varying dimensions.

After a boule of desired size is formed, the flame is shut off and theboule and apparatus are allowed to cool after which the boule is brokenaway from the rod 28.

As pointed out above, crystals of substantially pure rutile may varyfrom deep blue-black to light straw or nearly water-white colordepending upon the degree of oxygen deficiency. As produced, the bouleusually has a more or less frosted outer surface, and when split theinterior surfaces of the pieces are vitreous and shiny and may evenpossess a metallic luster. When formed, regardless of whether it is ofsubstantially pure rutile or whether it contains a coloring oxide, theboule is usually oxygen deficient and is characterized by a deepblueblack metallic color.

In the present invention, the oxygen deficient rutile single crystal isheated at elevated temperatures in an oxidizing atmosphere until asutficient amount of oxygen has been incorporated into the crystallattice to produce a predetermined desired color. As the crystal latticebecomes more nearly satisfied with respect to oxygen, the blue colorbecomes lighter and the transparency of the crystal increases until theblue color disappears entirely and the crystal is a straw white.

As hereinafter described, the invention also contemplates and includes asecond heating in a reducing atmosphere in the event that the crystalcontains an excess of oxygen over that desired for a particular color orother property. For example, if the oxidizing treatment is carried sofar that the crystal is of too light a color, the color may be darkenedby reduction.

As measured on a Beckman spectrophotometer, the color of an oxygendeficient rutile crystal has a dominant Wave length between about 480millimicrons and about 575 millimicrons, the dominant Wave length ofdarker crystals being near the lower end of the range and the dominantwave length of the lighter, more fully oxidized crystals being nearerthe upper end of the range. As the crystals are oxidized, thetransparency also increases. For example, a blue crystal 2.5 millimetersthick has a light transmission of about 25% while the same crystal whenfully oxidized has a light transmission of about 70% measured on aBeckman spectrophotometer.

The temperature at which the initial oxidizing heating should be carriedout should be within the range from about 650 C. to about 1500 C. It hasbeen found that at temperatures much below 650 (3., oxygen will not beappreciably incorporated into the rutile single crystal. It has alsobeen found that there is a decided loss in brilliance, luster and firein the rutile crystals when oxidized above 1500 C. and at thattemperature, the rate of oxygen incorporation into the rutile crystal isexcessively rapid and difiicult to control. Preferably the oxidizingheating should be carried out at about 1100" C. to 1300 C.

The oxidizing atmosphere is supplied by means of an oxygen-containinggas, for example air, in which the rutile single crystal is heated.Preferbaly, the heating should be carried out in a stream of oxygen orair enriched with respect to oxygen.

The heating should be continued for the length of time required toproduce the desired degree of oxidation as evidenced most convenientlyby the color of the crystal. After a slight amount of oxygen has beentaken up by the oxygen deficient crystal of substantially pure rutile,its color changes from the original blue-black to a deep blue. Furtherheating results in increased oxygen absorption by the crystal, the colorcorrespondingly changing to lighter shades until finally the crystal isfully oxidized as evidenced by loss of all blue tone and substantiallypure rutile exhibits a straw-white color. The color of the fullyoxidized crystal of substantially pure rutile is not water-white but maybe described as white with a straw tone or termed for convenience,straw-white.

It is not always possible when practicing the invention to achieveexactly the desired blue color. At the elevated temperature, the colormay differ from that when the crystal is cold. The oxidation isprogressive and slight differences in size of the crystals causeschanges in degree of oxidation with time. Minor impurities also mayaffect the oxidation rate, e. g. rutile boulescontaining the maximumtolerable amount of SiOz apparently reoxidize much more slowly thanboules having low SiOz content.

It may be found that a slight excess of oxygen has been incorporatedinto the crystal over that desired to obtain a certain color orproperty. This excess may be removed by subjecting the crystal to asecond, reducing heating. This second heating should be carried out attemperatures between about 500 C. and 1000 C., preferably 600 C. to 800C., in a reducing atmosphere, preferably a stream of hydrogen or amixture of hydrogen and an inert gas. Carbon monoxide may be employed,but it is not as effective as hydrogen. Below about 500 C., the crystalwill not give up oxygen while above about 1000 C. the liberation ofoxygen is too rapid for satisfactory close control.

When the rutile contains a coloring oxide, as for example alumina oroxides of cobalt and nickel, the oxidizing treatment causes theblue-black color to disappear so that characteristic color due to thecoloring oxide becomes apparent. By controlling the amount of oxidation,crystals can be produced in which the color due to the coloring oxide iscombined with the blue shades due to oxygen deficiency.

In order to achieve the best results, care should be exercised to avoidas much as possible alternating oxidizing and reducing treatmentsbecause repeated heating and cooling produces internal strains withinthe crystal so that it becomes brittle and will tend to shatter when anattempt is made to cut or shape it into some desired form. With thescope of the invention, those skilled in the art will quickly learn howto control the initial oxidizing treatment with a minimum of requiredsubsequent reducing treatment.

If desired, the whole boule as originally formed may be treatedaccording to the present invention but so to do will requireconsiderable prolongation of the treatment because of the relativelylarge size of the boule. It may, therefore, be desirable to cut theboule into a desired shape, for instance, a plate, or cube, or prism,prior to the initial oxidizing treatment. If the boule is suspected ofcontaining fractures, the presence of which cannot be readily detectedwhen the color is deep blueblack, it may be found expedient to carry outthe first oxidizing heating until the boule is fully oxidized and atwhich time, its light color will readily permit detection of fracture,then to cut the boule into the desired shape with reference to anyfractures found and to then subject such shapes to the second reducingheating.

It was found that the hardness of substantially pure rutile singlecrystals prepared according to the present invention is greater thanthat previously reported for rutile, viz. 6.5 on the Moh scale. Thefollowing table shows representative hardness values on the Moh scaleobtained for rutile crystals prepared according to the precentinvention.

Hardness Moh Scale Type of Crystal Parallel to ii iy Axis u ar to 0 AxisWhite (Straw tone)... 7. 5-8. 0 7. Light blue 6. 5 7 Deep blue 6. 0 70-7. 5

various changes and modifications can be made without departin from thespirit of the invention or the scope of the appended claims.

What is claimed is:

l. A method for the preparation of a rutile single crystal boule whichcomprises progressively fusing a finely divided, substantially pure TiOzin an oxidizing flame produced by the combustion of hydrogen and oxygenwherein a minor portion of said oxygen is introduced into the core ofthe flame and a major portion of said Oxygen, constituting with theminor portion an excess over that required for the combustion of thehydrogen, is introduced into the flame at the outer surface thereofforming an enveloping oxidizing atmosphere around said flame.

2. A method according to claim 1 in which the finely dividedsubstantially pure TiOz is introduced into the core of the flameentrained in the minor portion of the oxygen.

3. A method for the preparation of a rutile single crystal boule whichcomprises progressively fusing a finely divided substantially pure TiOzin an oxidizing flame produced by the combustion of hydrogen and oxygenwherein a minor portion of said oxygen insufficient for the completecombustion of the hydrogen is introduced into the core of the flame anda major portion of said oxygen, constituting with the minor portion anexcess over that required for the complete combustion of the hydrogen,is introduced into the flame substantially concentrically with said coreto complete the combustion of the hydrogen and provide an oxidizingatmosphere.

4. A method according to claim 3 in which at least 8.5 parts of oxygenare added for 9.5 parts of hydrogen by volume.

5. A method according to claim 3 in which the ratio of oxygen tohydrogen by volume is substantially 1 to 1.

6. A method for making a single crystal of rutile capable of having alight transmission for a 2.5 millimeter section of at least 25% whensubjected to a subsequent oxidizing heat treatment, comprisingperiodically passing axially through a flame and melting thereinpowdered titania, accumulating and crystallizing the titania so meltedas a single crystal on a support aligned axially with such flame, movingsuch flame and support apart axially of such flame, and maintaining anoxidizing atmosphere about said titania during fusing and crystallizing.

7. A method for making a single crystal of rutile having a lighttransmission for a 2.5 millimeter section of at least 25 comprisingperiodically passing axially through a flame and melting thereinpowdered titania, accumulating and crystallizing the titania so meltedas a single crystal of rutile on a support aligned axially with suchflame, moving such flame and support apart axially of such flame,maintaining an oxidizing atmosphere about said titania during fusing andcrystallizing, and subsequently heating the rutile crystal in anoxidizing atmosphere at a temperature between 650 C. and 1500 C.

8. A process for lightening the color of a dark single crystal ofsynthetic rutile to render it suitable for use as a gemstone, whichprocess comprises heating such a crystal at a temperature between about650 C. and about 1500 C. in an atmosphere of oxygen, and arresting suchheating when the desired lighter color is obtained.

9. A process for correcting overoxidation of a single crystal ofsynthetic rutile which comprises heating said crystal in an atmosphereof a reducing gas at a temperature between about 500 C. and 1000 C.

10. A process for making a single crystal of synthetic rutile of gemquality comprising periodically passing small amounts of powderconsisting essentially of titania axially down through an oxy-hydrogenflame onto a support to melt such powder; causing the powder so meltedto crystalline on said support as a substantially black and opaquesingle crystal of rutile; and then converting said single crystal to alighter color by heating said crystal at a temperature above 650 C. andbelow 1500 C. in an oxidizing atmosphere of oxygen, and arresting suchheating when the desired lighter color is obtained.

11. As an article of manufacture a highly refractive, flame-formedmonocrystalline mass of rutile adaptable for the preparation of gems andoptically useful objects having a color characterized by a dominant wavelength of about 480 millimicrons with a light transmission of about 25to a dominant wave length of about 575 millimicrons with a lighttransmission of about said light transmission being measured by aspectrophotometer at the dominant wave length through a section 2.5millimeters thick.

12. As an article of manufacture a highly refractive, flame-formed,monocrystalline mass of rutile adaptable for the preparation of gems andoptically useful objects having a color characterized by a dominant wavelength of about 575 millimicrons and a light transmission of about 70%through a section 2.5 millimeters thick as measured by aspectrophotometer.

References Cited in the file of this patent Websters New InternationalDictionary," 2nd ed. Unabridged, 1941, page 2190. G. & C. Merriam Co.,Springfield, Mass.

Danas: A Textbook of Mineralogy, 4th ed., 1932, pages 498, 499. JohnWiley & Sons, Inc., N. Y,

11. AS AN ARTICLE OF MANUFACTURE A HIGHLY REFRACTIVE, FLAME-FORMEDMONOCRYSTALLINE MASS OF RUTILE ADAPTABLE FOR THE PREPARATION OF GEMS ANDOPTICALLY USEFUL OBJECTS HAVING A COLOR, CHARACTERIZED BY A DOMINANTWAVE LENGTH OF ABOUT 480 MILLIMICRONS WITH A LIGHT TRANSMISSION OF ABOUT25% TO A DOMINANT WAVE LENGTH OF ABOUT 575 MILLIMICRONS WITH A LIGHTTRANSMISSION OF ABOUT 70%, SAID LIGHT TRANSMISSION BEING MEASURED BY ASPECTROPHOTOMETER AT THE DOMINANT WAVE LENGTH THROUGH A SECTION 2.5MILLIMETERS THICK.